Add support to the new backend system, including customization

Signed-off-by: ivgo <ivgo@spreadgroup.com>

.changeset/great-baboons-allow.md

@@ -0,0 +1,33 @@
+---
+'@backstage/plugin-vault-backend': minor
+---
+
+Added support for the [new backend system](https://backstage.io/docs/backend-system/).
+
+In your `packages/backend/src/index.ts` make the following changes:
+
+```diff
+  import { createBackend } from '@backstage/backend-defaults';
+  const backend = createBackend();
+  // ... other feature additions
++ backend.add(import('@backstage/plugin-vault-backend');
+  backend.start();
+```
+
+If you use the new backend system, the token renewal task can be defined via configuration file:
+
+```diff
+vault:
+  baseUrl: <BASE_URL>
+  token: <TOKEN>
+  schedule:
++   frequency: ...
++   timeout: ...
++   # Other schedule options, such as scope or initialDelay
+```
+
+If the `schedule` is omitted or set to `false` no token renewal task will be scheduled.
+If the value of `schedule` is set to `true` the renew will be scheduled hourly (the default).
+In other cases (like in the diff above), the defined schedule will be used.
+
+**DEPRECATIONS**: The interface `VaultApi` and the type `VaultSecret` are now deprecated. Import them from `@backstage/plugin-vault-node`.